Diatomaceous earth has been used for many years in a number of applications utilizing its absorptive properties and its filtration properties, among other applications. Diatomaceous earth ore is a naturally occurring ore that is fairly easily crushed or crumbled into a fine powder. Diatomaceous earth consists primarily of the skeletal remains of diatoms, which is a type of algae, and includes primarily silica, along with some minor amounts of sodium, magnesium, and iron. The percentages of the various elements may vary depending on the source or collection point of the diatomaceous earth, but generally the silica (in an amorphous form) constitutes over 85% by weight of the diatomaceous earth.
Diatomaceous earth has been used for many years as a filter aid due to its high porosity and because its porosity can be adjusted by modifying the particle size of the final diatomaceous earth product. Conventional processes used to produce diatomaceous earth filter aids typically begin with a crushing and milling step in which the diatomaceous earth ore is milled in an open circuit to a median particle size of between 10 and 20 micrometers. The milled ore is then sent to a calciner where the ore is heated to temperatures greater than about 1000° C. In the past, the calcining step has been done both with and without the addition of a fluxing agent. In conventional processes, the discharge of the calciner is typically refined or milled to attain the desired final particle size prior to packaging.
In conventional processes, some of the amorphous silica of the diatom frustules is converted to crystalline silica in the form of cristobalite. Crystalline silica is considered to be a health risk by many, especially in a respirable form (i.e., particle size smaller than 10 micrometers). Diatomaceous earth filter aids produced through conventional methods contain greater than 1% by weight crystalline silica and generally contain 50-75% by weight crystalline silica. There has been an industry focus on efforts to reduce the amount of crystalline silica in diatomaceous earth filter aids and particularly the respirable particles of crystalline silica. Unfortunately, these efforts heretofore have not been successful at developing a method capable of producing filter aids with a diversity of permeabilities.
It is known that the conversion from amorphous silica to crystalline silica occurs at high temperatures, whether flux is added or not, and that the conversion is accelerated when a sodium-based flux is added. Accordingly, efforts to reduce crystalline silica formation have included attempts to calcine without a fluxing agent, to calcine with a non-sodium based flux, to reduce the time the diatomaceous earth is exposed to high heat in the calciner (so called “flash calcining”), to eliminate the calcining step altogether, or some combination of the above. However, to produce diatomaceous earth filter aids having a range of permeabilities (i.e., ranging from 0.01 darcy to greater than 10 darcy, including for example 20 darcy or 30 darcy), a variety of processes would be implemented to produce the various filter aids.
For example, diatomaceous earth filter aids produced without calcining have a restricted permeability range between 0.01 darcy and 0.10 darcy. The addition of heat, such as in a conventional calcining step, causes some sintering of the diatoms, drives off the water of hydration, reduces the specific surface area of the particles, and results in filter aids with permeabilities between 0.05 darcy and 1.0 darcy, but also produces some crystalline silica. In order to obtain still higher permeabilities (i.e., greater than 1.0 darcy), flux is added prior to the calcining step to form a vitreous phase during calcining that agglomerates the individual diatoms and diatom pieces to form much coarser particles. Diatomaceous earth filter aids made using a flux can have permeabilities of between 0.4 darcy and 30 darcy.
Depending on the permeability level desired and the level of crystalline silica tolerable in the final product, conventional diatomaceous earth manufacturers would run one of the above processes, sometimes being limited in permeability ranges by the tolerable crystalline silica. Accordingly, a single manufacturing process capable of producing a complete range (high and low permeability) of diatomaceous earth filter aids having less than 1% by weight crystalline silica has not been available.
At least two prior attempts to provide such a manufacturing process have been unsuccessful for different reasons. One such attempt was a flash calcine process where the diatomaceous earth was calcined by reducing the residence time of the diatomaceous earth in the hot zone of the calciner. As the formation of crystalline silica is time and temperature dependent, the theory was that by reducing the residence time the formation of cristobalite could be avoided even when a flux was used. However, no one has yet been able to make this concept economically viable on a commercial scale. Another such attempt included using an alternative fluxing agent, such as substituting a potassium-based flux for the sodium-based flux. While potassium-based fluxing agents have been successfully used to produce diatomaceous earth filter aids, it has not been shown to be able to produce medium or high permeability filter aids.
The present invention provides methods capable of producing diatomaceous earth filter aids having a large range of permeabilities and having less than 1% by weight crystalline silica. Additionally, the present invention provides diatomaceous earth filter aids having less than 1% by weight crystalline silica and having permeabilities greater than 1.0 darcy, which were heretofore unattainable.